In the field of circuit design, designers often need resistors that can predictably change resistance based on an input control voltage applied to the resistors. Basically, a voltage-controlled resistor is a three-terminal device that varies the resistance between two terminals relative to a voltage applied to the third terminal. A wide variety of electronics use voltage-controlled resistors. Examples include variable-gain amplifiers, voltage-controlled oscillators, automatic gain controllers, low-distortion tunable filters, audio dynamic range compression, and electronically controlled phase advance and retard circuits.
Field-effect transistors (FETs) can be used as voltage-controlled resistors. The resistance change for an FET can be more than one order in magnitude. The resistance change, however, is not linear over the entire range. Instead, FETs can operate linearly only in a relatively low and small voltage region (typically around 1V) with a linearly tunable resistance change spanning less than one order in magnitude. Several techniques can be used to improve parameters such as linearity, voltage operation region, and linear tunable range of the resistance. These techniques, however, add complex circuitry or improve only a sub-set of these parameters.
Alternatively, a photo-resistor can be combined with a light-emitting diode, and the combination can be used as a voltage-controlled resistor. The combination, however, results in a very bulky device, which consumes large amounts of power. Additionally, the device suffers from a relatively low linearly tunable resistance span. Further, since these devices are made from discrete components, it becomes difficult to produce identical pairs of devices. Therefore feedback techniques are not feasible.
Digital potentiometers can also be used as voltage-controlled resistors. Digital potentiometers, however, suffer from several drawbacks. Some of them include: (1) digital quantization nature of potentiometers hinders possible resolution of tunable resistance values, which worsens as the resistance span increases; and (2) use of multiple digital components (e.g. microcontrollers or microprocessors) adds complexity while reducing practicality.
Accordingly, there is a need for a voltage-controlled resistor having a resistance that is linear over a large voltage range. Additionally, there is a need for a voltage-controlled resistor that can be operated linearly over a wide range of positive and negative voltages. Further, there is a need for a voltage-controlled resistor that does not require complex circuitry and that is compact and relatively inexpensive to fabricate.
This section provides background information related to the present disclosure which is not necessarily prior art.